The radiation of edge-emitting diode lasers is highly divergent in a direction perpendicular to the waveguide plane (vertical direction, “fast axis”) and has a comparatively broad wavelength spectrum. In addition, the wavelength spectrum typically depends on further parameters, such as the temperature. As a result, the wavelength spectrum depends on the power supplied by the laser.
According to the state of the art, the wavelengths can be limited and stabilized by means of internal or external wavelength-selective elements or structures. An external limitation and stabilization of the wavelength is achieved as a result of spectrally selective feedback of the emitted radiation into the diode laser. An example is a so-called external cavity diode laser (ECDL) where feedback is done by means of spectrally selective reflection e.g. on surface gratings. This, however, has the drawback that additional optical elements are required and miniaturization is made difficult.
Another way to achieve spectrally selective feedback is the use of volume Bragg gratings (also referred to as VBGs). The advantage of using such VBGs is that compact, wavelength-stabilized laser beam sources can be implemented. For example, DE 10 2011 006198 A1, US 2005/0207466 A1, US 2006/0251143 A1, U.S. Pat. No. 7,397,837 B2 and U.S. Pat. No. 7,545,844 B2 disclose how to place a volume Bragg grating in the (collimated) laser beam. However, said arrangements are disadvantageous in that the VBG is arranged within the main optical path (i.e. the path along which the laser radiation is coupled out) thereby receiving high optical energy which may result in a wavelength drift with higher radiation energies. This small shift of the peak wavelength of the locked diode lasers arises from a heating of the VBG with increasing power which causes a slight change of the locking wavelength. Furthermore, prior art laser diode systems use VBGs that provide a fixed (reflectivity) percentage for the feedback signal resulting in high intensity feedback levels for large currents of respective (current-driven) laser diodes. That is, the level of the feedback signal is higher than needed (for large diode driving currents) in order to realize wavelength stabilization thereby reducing the overall output power of the laser diode system. On the other hand, if the feedback is optimized for large diode driving current, it will be too small for low currents. Further, the VBGs in the prior art need to be adapted in their geometrical layout in order to achieve the necessary (low) reflectivity thereby causing unwanted detrimental effects such as diffraction. In addition, said laser diode arrangements suffer from reduction of the overall output power in that the used laser diode does not produce radiation being completely polarized. In fact, real laser diodes exhibit a typical degree of polarization for the main polarization direction ranging from approx. 80% to 95%. That is, the non-polarized portion of the emitted laser radiation vanishes within the laser system, e.g. at edge filters or polarizing filters.
Therefore, the object of the invention is to provide a diode laser with wavelength stabilization which overcomes the deficiencies in the prior art and which allows an increased overall output power.